1. Field of the Invention
The present invention relates to transmission power controls (or uplink power controls) of mobile stations (e.g. mobile phones) in mobile communication systems.
The present application claims priority on Japanese Patent Application No. 2009-231920, the content of which is incorporated herein by reference.
2. Description of the Related Art
In Long Term Evolution (LTE) (recently developed as 4G, i.e. Fourth Generation wireless network), base stations are able to communicate the allocation information and the interference information such as overload indicators (OI) and high interference indicators via backbone networks, so that base stations can cooperate together based on such information. Each sector uses an overload indicator (OI) to communicate the interference information to a neighboring sector in units of frequency blocks (i.e. minimum units of allocating radio resources or consecutive frequency bands). The interference information has three stages, denoted with numerals “0”, “1”, and “2”, wherein a higher interference level has a higher numeral.
A transmission power control using the interference information detected in neighboring sectors has been developed and disclosed in Patent Document 1 and Non-Patent Document 1, both of which refer to a method for determining transmission power controls in mobile stations. This method is implemented in accordance with standardization specifications of cellular systems, known as Ultra Mobile Broadband (UMB).
Specifically, Patent Document 1 teaches a method for determining power increase/decrease based on the overall interference information (i.e. the interference information regarding all frequency bands) which is calculated in connection with a sector having a minimum path loss (i.e. a power loss which occurs when radio waves move through space along a specific path) among sectors except for serving sectors (or serving sectors). A basic operation of transmission power control is implemented in accordance with a policy in which power is decreased or maintained (without change) with respect to a high level of interference whilst power is increased or maintained with respect to a low level of interference. A determination as to whether to increase or decrease power is made in light of a probability, which is calculated based on a present power level and a difference between a path loss of a serving sector and a “minimum” path loss of a certain sector which differs from the serving sector. Non-Patent Document 1 teaches three stages, denoted with numerals “0”, “1”, and “2” in IoT-Level (where “IoT” stands for “Interference over Thermal”), as the interference information. A transmission power control using a path loss difference is able to reduce interference between cells and interference between sectors.                Patent Document 1: Japanese Patent Application Publication No. 2008-92545        Non-Patent Document 1: 3GPP2 C. S0084-002-0, “Medium Access Control Layer For Ultra Mobile Broadband (UMB) Air Interface Specification”        
The above method determines a power increase/decrease in a mobile station based on a difference between a path loss occurring between a mobile station and a serving sector and a path loss occurring between the mobile station and another sector which differs from the serving sector. In some mobile communication systems such as an LTE-based mobile communication system, a base station is unable to obtain the information (e.g. a path loss occurring between a mobile station and a neighboring sector) regarding other sectors which differ from a serving sector; hence, it is impossible to estimate a path loss with respect to other sectors. In this case, it is impossible to determine a power increase/decrease with respect to a mobile station, in other words, it is impossible to adopt the above method for determining transmission power in a mobile station.